Today there are various cellular radio or mobile telephone systems in use and being planned in which the geographical area covered by the system is divided into smaller separate radio areas, that is, cells in such a manner that a radio telephone or a mobile telephone in a cell communicates with a fixed network via a fixed radio station placed in the cell. The mobile telephones in the system can move freely in the area of the system from one cell to another. One such system is the digital mobile telephone system GSM (Global System for Mobiles).
In connection with cellular radio networks, it is known that a home location register stores the location information and subscriber data of the radio telephones registered to the network constantly into a memory means, such as RAM, from which the data will volatilize for example when the power supply of the home database is cut off, and at regular intervals into a non-volatile memory means, for example, into a hard disk on which the data is retained permanently. When the home database occasionally has to restart itself, the location information and subscriber data stored into the volatile memory will disappear. Then the home database has to request all the visitor databases of the mobile communication system, such as visitor databases of the radio telephone network, to perform the location updating of the subscribers registered to this home database in order that the home database would be able to update the location information of its subscribers. The home database needs the location information of the subscribers so that it is able to submit the correct location of the subscriber to the mobile services switching center intending to transmit a call to this subscriber.
Some prior art solutions are disclosed in GSM Specification "prETS 300 599, (GSM 09.02 version 4.9.0), November. 1994, European digital cellular telecommunications system (Phase 2); Mobile Application Part (MAP) specification, ETSI, p. 317 to 468."
According to the specification above, the recovery is performed after restarting the home location register (HLR) or the visitor location register (VLR).
The restart of the home location register HLR takes place in such a manner that the subscriber data of the home location register HLR in the visitor location register VLR is set to a state in which the visitor location register VLR transmits a registration request to the home location register HLR only when a certain impulse relating to the subscriber is received. This impulse can be a subscriber registration, an incoming call to a subscriber or a subscriber's call attempt. The information in the visitor location register/visitor database is the location information of the subscriber and data on the services permitted to and activated for the subscriber.
The restart of the visitor location register/database VLR takes place in such a manner that the data of the subscribers registered to the visitor location register is deleted from the visitor location register, whereby the visitor location register VLR sends a registration request to the home location register HLR when it detects a certain impulse relating to the subscriber, for example, a subscriber registration or an incoming call to the subscriber.
Above and in the following, home location register is considered to be similar to home database and visitor location register is considered to be similar to visitor database. Both are databases into which are stored the location of the subscriber with a certain accuracy and the subscriber data.
A problem of prior art solutions is that they do not ensure the accessibility of the subscribers in all situations even though the subscriber has a valid registration at some visitor database VLR.
The first example of a problem situation of the prior art is a situation in which the home database HLR loses data on which home database VLR the subscriber is registered to at the moment of observation. This fault situation may be caused by the home database getting suddenly damaged, for example. In that case the subscriber cannot be reached via the home database HLR before the subscriber has next time a radio contact with the system, whereby the registration data of the subscriber will be updated to the home database.
The second example of a problem situation of the prior art is a situation in which the deletion of registration from the previous visitor database VLR has not been successful in connection with handover/roaming/handoff, that is, in a situation in which the subscriber has moved from the service area of the original visitor database to a service area of a new visitor database. In a situation like this the deletion of the subscriber from the first visitor database is to be done so that the home database sends a deletion message to the visitor database with which the registration of said subscriber to the visitor database is to be deleted. If this deletion operation is not successful, or if the data recovery method is not capable of amending the situation, a difficult situation will arise. Then calls made from the area of the visitor database to the subscriber, for example, will fail as the visitor database pages said subscriber in its own service area although in actual fact, said subscriber is already registered to some other visitor database. The paging visitor database is not aware of the situation, and attempts to page the subscriber in its own service area and establish a call to a subscriber that is in actual fact not in the service area of the visitor database.
The third disadvantage of the prior art solutions is caused by that since at the restart of the visitor database VLR the data of the visitors is deleted from the active memory of the database, in order to operate well the method requires subscriber search or paging by broadcasting paging messages over the radio path to all subscribers. Subscriber paging through the radio path loads the radio path considerably between the base station and mobile station of the mobile communication system and thus consumes radio path capacity. Subscriber paging is used for example in connection with call establishment for finding a B subscriber in the area of the visitor database VLR when the data of said B subscriber e.g. the location information of the subscriber is not found in the visitor location register indicated by the home database. The problem is caused by that the visitor database has for some reason lost the data of the mobile station but this data is still in the home database of said subscriber. Then the home database knows the location of the subscriber only at the accuracy of the visitor database VLR, but not at the accuracy of the location area (LA), whereby the visitor database has to page said subscriber from its own service area by means of messages sent on the radio path. They naturally load the radio path unnecessarily.
The fourth disadvantage of the prior art solutions is caused by that recovery operates in them triggered by subscriber units. Then the loading caused by recovery in the nodes and between the nodes is uncontrolled because the subscribers start the recovery by their own activity, e.g. when registering into the network or when sending messages. Therefore the start of recovery may be periodical and bound to the time of the day and the day of the week, whereby the recovery of the data of several subscribers may start relatively at the same time, for example, when several subscribers register to the network at the beginning of a working day. This kind of recovery, started by the movements or the activity of the subscribers, causes uncontrolled loading to the system and because of the inconvenience it causes to subscribers, it is not very suitable to be used periodically without a detected fatal fault situation.
In one solution of the prior art, recovery delay is shortened by forcing the subscribers to register to the mobile communication system and its databases as often as possible even if the radio subscriber does not change the base station. A drawback of this procedure is that it loads the radio path because then the subscribers will send registration messages of their own at desired intervals on the radio path to base stations from which they are forwarded to exchanges and databases.